Summers immediately agreed to the request Monday afternoon. A spokeswoman for King said he was at an out-of-state fundraiser and unable to respond immediately to the request.

Dill said her idea is for all three candidates to release their tax returns at the same time through a third party. Dill, who sent letters to Summers and King on Monday, said releasing the tax returns would help the candidates "set the national standard for public accountability.''

The letter comes against a backdrop of a similar debate in the presidential race in which Republican Mitt Romney refused to release returns before 2010.